Background　Lacking of integrated and systematic pain assessment tools has been a serious obstacle of pain management in the intensive care unit（ICU） setting. Pain relief is of great importance not only for patients but also for their families. In order to provide the high quality of nursing service， a pain assessment and a timely pain relief have to be accomplished at first. However， no pain assessment equipment has been widely accepted or applied to patients with difficulty in communication.　Objective　This review analyzed and summarized the existent pain assessment tools for those critically ill patients with communication difficulty.　Content　Firstly we discussed the importance of pain assessment for critically ill patients. And then we introduced 6 different pain assessment tools including" behavior pain scale（BPS）"， "face， legs， activity， cry， consolability behavioural tool（FLACC）"， "nociception coma scale（NCS）"，"non-verbal pain scale（NVPS）"， "the critical-care pain observation tool（CPOT）"and "the pain assessment in advanced dementia scale（PAINAD）". Finally we discussed the limitations of those pain assessment tools.　Trend　In our country， the critically ill patients' pain assessment was just in the start-up phase. Regular use of pain assessment tools can be helpful to avoid the state of pain with lacking of analgesia caused by over sedation ， and also to improve the compliance of the patients.